This invention relates to crystal resonator structure for measuring pressure in fluids and a method of manufacture thereof.
One of the present techniques used in measuring pressure in caustic environments, such as in deep oil and gas wells, involves the use of a quartz crystal transducer apparatus which includes a circular resonator section peripherally supported within a hollow, cylindrical housing formed as an integral part of the resonator section. See U.S. Pat. Nos. 3,617,780 and 3,561,832. The resonator section of such apparatus is caused to vibrate by oscillatory electrical signals applied to electrodes placed on the resonator section. The frequency of vibration of the resonator section varies with variation in radially-directed stresses in the resonator section caused by pressure on the housing. Variation in the frequency of vibration of the resonator section thus affords a measure of the pressure to which the housing is subjected.
An improvement upon the device described above was recently developed to minimize the effect of temperature transients on the pressure measurements. See U.S. Pat. No. 4,550,610, for example. This improvement involves the placement of thinner sidewall sections at selected locations in the housing so that a non-uniform stress is produced in the resonator section. This non-uniform stress is then utilized to reduce scale factor dependence upon temperature.
Another piezoelectric device used to make temperature-corrected pressure measurements is disclosed in U.S. Pat. No. 4,562,375. This device also employs an outer housing in which is disposed a circular resonator section or pellet. The resonator pellet is held in place within the housing by two or more bridges which extend between the interior housing sidewall and locations on the periphery of the pellet, with the rest of the periphery being separated from the sidewall by a gap or "interval".
Providing fairly large devices of the designs described above, including relatively large diameter hollows or bores in the device housing, simplifies manufacturing and yields a resonator with desirable electrical performance characteristics. However, because of the size, each device is fairly expensive to manufacture because of the expense of finding or producing large pieces of suitable quartz. Also, the devices cannot be heated or cooled too rapidly if thermal fracturing is to be avoided; and stabilization of the devices to avoid errors in measurement of pressure after temperature or pressure changes is quite time consuming. Finally, it has been reported that with the devices of the above-described designs, large increases in resonator resistance and transducer pressure hysteresis occur as attempts are made to increase the operating temperature and pressure range of the transducers.
Transducers of smaller size have been fabricated and are considered desirable because, among other reasons, costs are less since smaller pieces of defect-free quartz are readily available. Also, higher operating temperatures and pressures can be measured without an increase in resonator resistance and transducer pressure hysteresis. But, because of the reduced size, and the small size of the bores, it has proved difficult to obtain precision in contouring the surfaces of the resonator section a desired feature of resonator pressure transducers.